The present invention relates to a method for separating various isotopes of an element in which the molecules of the element in question form an isotopic compound gas and in which the molecules of the pure molecular gas or of the molecular gas mixed with an additional gas are selectively excited with the aid of a laser, the excited molecules being separated by either physical or chemical means.
It is known to effect the separation of isotopes by selective excitation with laser light, but it has not as yet been possible to develop a process based on this principle which could be used in large-scale operations. The presently actually employed separation processes, such as diffusion, centrifuging, etc., effect separation on the basis of the small difference in mass between the various isotopes. Since this difference in mass is very slight, the separation must be effected in numerous stages so as to achieve a sufficient concentration of the respective isotope. Thus, for example, it requires more than 1000 stages in the uranium enrichment process by diffusion to enrich the U.sup.235 isotope, suitable as a nuclear fuel material, from its natural concentration of about 0.7% to a concentration of 3% required in the reactor.
In contradistinction thereto, isotope separation by selective enrichment by means of lasers offers the possibility of realizing sufficient enrichment in one or a few stages. In principle, such isotope separation can be based on the atoms of the respective element or on molecules formed from these atoms. After the selective enrichment, the excited atoms or molecules can be separated from the nonexcited molecules by various types of physical or chemical processes.
Isotope separation with the aid of lasers has already been proposed and is performed, as disclosed in U.S. Pat. No. 3,443,087 to J. Robieux and J. M. Auclair, by selectively exciting U.sup.235 molecules with laser radiation in a vacuum chamber filled, for example, with UF.sub.6 gas, then ionizing these molecules by irradiation with ultraviolet light, and thereafter deflecting the ionized molecules out of the gas by means of electric or magnetic fields.
This creates the difficulty, however, that the first step of selective excitation requires a comparatively very small amount of energy, amounting to only 0.06 eV to 0.2 eV, and that the ionization energy required for the second step does not have a precisely defined value. This is explained by the fact that the energy introduced is utilized not only for ionization but distributes itself to many energy levels of the molecule and may also effect dissociation. By irradiating with monochromatic ultraviolet light, even with selection of the most favorable wavelength, not only the excited molecules but, to a considerable extent, also the non-excited molecules of the other type of isotope are ionized. To reduce this difficulty, the excitation has to be effected in several stages and this involves great losses of laser energy and correspondingly a great amount of apparatus.
According to another known isotope separation process involving selective excitation with lasers are described in German Offenlegungsschrift [Laid-Open Application] No. 1,959,767, by K. Gurs, the molecules, e.g. UF.sub.6 are excited isotope-specifically in their vibration rotation spectra and are then chemically converted in a suitable reaction mixture. Due to the excitation, the activation energy required for the chemical reaction is reduced and the reaction speed is increased under certain conditions by the Boltzmann factor e.sup..delta..sup.E/kT, where .DELTA.E constitutes the quantum energy introduced. For quanta of the CO.sub.2 laser, the Boltzmann factor has a value of 100 at room temperature (T= 300 K).
Both above-mentioned isotope separation processes employing lasers required as a first step a selective excitation of the molecules. This is possible only, due to the great width of the vibration rotation bands relative to the isotope shift, if the bands are separated into individual vibration rotation lines. However, even at low pressure the individual lines will overlap to a greater or lesser degree depending on temperature and wavelength so that simultaneously with the molecules of the desired type of isotopes other molecules are always excited along. It is not possible to lower the temperature much below room temperature because otherwise the UF.sub.6 gas would freeze practically completely and thus the quantity of gas to be converted during isotope separation in the process would not be sufficient. Due to the incomplete separation of the vibration rotation lines the efficiency of the last described process of isotope separation based on selective excitation of the molecules and subsequent chemical reaction of the excited molecules is also limited, the separation factor being reduced.
It has also been suggested that it may be possible to base the isotope separation with lasers on uranium atoms, this being disclosed in German Offenlegungsschrift No. 2,312,194, by J. Nebenzahl and M. Levin and German Offenlegungsschrift No. 2,353,002, by R. Levy and G.S. Janes. These processes include a multistage photoionization of uranium atoms. In this process uranium vapor is produced and this vapor is irradiated by a tunable narrowband dye laser and a further light source. The quantum energy of the dye laser and of the additional light source are less than the ionization energy of the uranium atoms; however their sum is greater than the uranium atom ionization energy.
The wavelength of the dye laser must be selected so that only the U.sup.235 isotope is excited. If such an excited atom absorbs a quantum of the second light source, it is ionized. Since such ionization is isotope-specific, the separation of the isotopes can be effected by deflecting the ions in an electric or magnetic field.
Due to the short life time of the level excited in the first stage and the low possibility of transition to the ionization continuum, the quantum yield for photoionization is low. Since furthermore the efficiency of the required light sources, i.e., the dye laser and high pressure lamps, is also low, it is not possible with such process to realize a substantial reduction of the energy requirement compared to the gas centrifuge process already in use. There is the added drawback that the uranium must be present in its vapor phase. The high temperature of more than 2300.degree. C required for this conversion already produces great technical difficulties. No material is known at this time which is chemically resistant to uranium metal at such high temperatures.